At the end of this comedy of errors, a late punchline forced a reprise which Newcastle could not need less.

Four minutes from the end of a second half in which their best - and the home defence's worst - efforts should have seen them through, United's own frailties allowed Sebastian Larsson to deprive them of a fourth-round place outright.

And that the overall balance of play, and 10-man Birmingham's own injury crisis, suggests the Magpies will have the last laugh a week on Wednesday will bring them scant consolation.

Having watched his defensive understudies' inexperience exposed for DJ Campbell's opener on the quarter-hour, Glenn Roeder saw Steven Taylor punish similar sloppiness at the other end with a 40th-minute equaliser.

And with Birmingham reduced to 10 men by Radhi Jaidi's professional foul on Obafemi Martins, Kieron Dyer's 54th-minute strike seemed certain to settle the tie at the first attempt.

But despite the assistance of Matthew Upson (#6 million indeed!) and Co in the home defence, Newcastle failed to secure the buffer of a third goal.

And such was the fright provided by Larsson's late leveller, the final whistle brought relief rather than rejoicing to a packed away end.

Nowhere was the class gap between the clubs greater than on the terraces, where the barely 12,000 Brummies who turned up were embarrassed by United's travelling army.

But despite their squad being so decimated that Kazenga LuaLua - three weeks past his 16th birthday - was among their substitutes, Newcastle were a cut enough above the Blues to render Roeder mightily frustrated.

Perspiration may have outweighed inspiration at the outset, but Newcastle adapted their passing game to a gluepot pitch encouragingly enough.

All too quickly though, the disastrous dead-ball defending which dogged them at Everton a week earlier cost the Magpies dear again.

Their first mistake was in allowing Upson to head Larsson's deep left-wing corner back across goal.

Their second - failing to detect Campbell's presence - proved fatal, as the striker prodded home from barely four yards.

Newcastle's rookies were rattled, with Paul Huntington grateful to Shay Given for preventing his back header from becoming United's second own goal in 12 days.

Seconds earlier, Obafemi Martins had shot wide, but there was no denying the home side's impetus.

Nicklas Bentdner's power and Campbell's pace were problematic in open play, while Gary McSheffrey hit the side netting with a free-kick.

The sight of Martins stinging Maik Taylor's palms with a shot on the turn - and injury to Steven Kelly bringing Martin Taylor on as a makeshift right-back - brought encouragement.

And though Geordie optimism was tempered when more self-destructive defending almost gifted Blues another goal, the hosts would soon pay the price of their own double suicide attempt.

Not that Steven Taylor's heroic intervention at either end of the pitch can be downplayed.

First, he cleared off the line from Campbell after Huntington had allowed Bendtner to reach the byline and cut the ball back.

Then Taylor somehow stole in unnoticed to volley home James Milner's left-wing corner at the near post.

If that exasperated Steve Bruce, what happened next left him distraught and relieved in equal measure.

The agony arrived when Jaidi saw red for a blatant rugby tackle on Martins after failing to cut out Milner's long ball.

But Bruce's consolation was considerable, with referee Mike Dean making the marginal decision that Martins was felled fractionally outside the box.

Maik Taylor's save from Milner's free-kick confirmed that Birmingham had got off lightly.

The break brought changes by both sides, but no initial respite for the Blues.

Upson contrived to let Martins' pass between his legs and leave Dyer with only Taylor to beat.

And beat him he did with a right-foot cross shot from 12 yards.

The shame - and the wonder - was that Newcastle failed to then make the game safe.

Upson's agonies continued as he gifted Martins a chance which was dithered over long enough for Taylor to retrieve possession.

The keeper was also out smartly to prevent Dyer from reaching a ball from Nicky Butt as the Blues' defence sank without trace.

Granted, Given had to block with his legs when David Edgar's challenge rebounded goalwards off McSheffrey.

But Bendtner's departure through injury promised to grant Newcastle's rearguard real comfort in the closing stages, particularly as Birmingham's own defence remained rickety at best.

Not so.

United allowed Damien Johnson to swing in a cross which Huntington could only nod into the path of Larsson.

And Edgar's presence between Larsson and goal only unsighted Given as the Swede hammered home a shot on the turn.

From nowhere, panic descended on Roeder's troops like a fog.

Thankfully, Given remained resolute to deny Neil Kilkenny and Fabrice Muamba late on.

Page 2: Managers' view

Managers' view

Glenn Roeder last night rued the rookie defending and lack of ruthlessness which left Newcastle facing an unwanted FA Cup replay.

The United boss blamed inexperience within his back four for Birmingham's late equaliser in a thrilling third-round tie at St Andrews.

But Roeder's real frustration lay in United's failure to kill off the 10-man Blues by then.

"We had a one-goal lead with five minutes to go and Birmingham a man short, so I would have expected us to see the game out," said Roeder.

"Unfortunately, the equaliser showed the inexperience in our team today - we couldn't handle the cross into the box.

"So I'm not happy." Roeder added: "I knew it would be a rip-roaring cup tie. Birmingham are doing exceptionally well in the Championship, and on a muddy pitch it had the potential to be a difficult match. But we made it more difficult than it should have been.

"We're pleased as a club to be in the draw for the fourth round. And although we expect to win the replay, we have to make sure we do the job on the night."

Roeder could at least allay fears that Antoine Sibierski has joined United's lengthy injury list, which saw 16-year-old Kazenga LuaLua win a place on their bench yesterday.

Sibierski was withdrawn at half-time, but Roeder revealed: "He was feeling very unwell before the game - there is a bit of a virus around at the moment."

Had he been brought off the bench, LuaLua - brother of Lomana - would have been Newcastle's youngest-ever first-team player.

However, Birmingham boss Steve Bruce was left to reflect on the price his side paid for forcing a replay.